GETDENTS(2)                                                                Linux Programmer's Manual                                                                GETDENTS(2)

NAME
        getdents, getdents64 - get directory entries

SYNOPSIS
        int getdents(unsigned int fd, struct linux_dirent *dirp,
                    unsigned int count);
        int getdents64(unsigned int fd, struct linux_dirent64 *dirp,
                    unsigned int count);

        Note: There are no glibc wrappers for these system calls; see NOTES.

DESCRIPTION 
        These are not the interfaces you  are interested in.
        Look at readdir(3)  for the POSIX-conforming C library interface.
        This page documents the ¿bare kernel system call¿ interfaces. 

    getdents()
        The system call getdents() reads several ¿linux_dirent structures¿ 
        from the directory referred to by the open file descriptor fd
        into the buffer 
                    pointed to by dirp.
        The  argument ¿count¿ specifies the size of that buffer.

        The linux_dirent structure is declared as follows:

            struct linux_dirent {
               unsigned long d_ino;     /* Inode number */
               unsigned long d_off;     /* Offset to next linux_dirent */
               unsigned short d_reclen; /* Length of this linux_dirent */
               char           d_name[]; /* Filename (null-terminated) */
                                 /* length is actually (d_reclen - 2 -
                                    offsetof(struct linux_dirent, d_name)) */
               /*
               char           pad;       // Zero padding byte
               char           d_type;    // File type (only since Linux
                                         // 2.6.4); offset is (d_reclen - 1)
               */
            }

        d_ino is an inode number. d_off is the distance from the start of the directory to the start of the next linux_dirent. d_reclen is the size of this entire linux_dirent. d_name is a null-terminated filename.

        d_type is a byte at the end of the structure that indicates the file type. It contains one of the following values (defined in <dirent.h>):

        DT_BLK      This is a block device.

        DT_CHR      This is a character device.

        DT_DIR      This is a directory.

        DT_FIFO     This is a named pipe (FIFO).

        DT_LNK      This is a symbolic link.

        DT_REG      This is a regular file.

        DT_SOCK     This is a UNIX domain socket.

        DT_UNKNOWN The file type is unknown.

        The d_type field is implemented since Linux 2.6.4. It occupies a space that was previously a zero-filled padding byte in the linux_dirent structure. Thus, on kernels
        up to and including 2.6.3, attempting to access this field always provides the value 0 (DT_UNKNOWN).

        Currently, only some filesystems (among them: Btrfs, ext2, ext3, and ext4) have full support for returning the file type in d_type. All applications must properly handle a return of DT_UNKNOWN.

    getdents64()
        The original Linux getdents() system call did not handle large filesystems and large file offsets. Consequently, Linux 2.4 added getdents64(), with wider types for the
        d_ino and d_off fields. In addition, getdents64() supports an explicit d_type field.

        The getdents64() system call is like getdents(), except that its second argument is a pointer to a buffer containing structures of the following type:

            struct linux_dirent64 {
               ino64_t        d_ino;    /* 64-bit inode number */
               off64_t        d_off;    /* 64-bit offset to next structure */
               unsigned short d_reclen; /* Size of this dirent */
               unsigned char d_type;   /* File type */
               char           d_name[]; /* Filename (null-terminated) */
            };

RETURN VALUE
        On success, the number of bytes read is returned. On end of directory, 0 is returned. On error, -1 is returned, and errno is set appropriately.

ERRORS
        EBADF Invalid file descriptor fd.

        EFAULT Argument points outside the calling process's address space.

        EINVAL Result buffer is too small.

        ENOENT No such directory.

        ENOTDIR
              File descriptor does not refer to a directory.

CONFORMING TO
        SVr4.

NOTES 
        Glibc does not provide a wrapper for these system calls;
            Call them using syscall(2). 
            You will need to define the linux_dirent or
            linux_dirent64 structure yourself.

        However,
            you probably want to use readdir(3)  instead. 

        These calls supersede 取代 readdir(2).

EXAMPLE
        The program below demonstrates the use of getdents(). The following output shows an example of what we see when running this program on an ext2 directory:

            $ ./a.out /testfs/
            --------------- nread=120 ---------------
            inode#    file type d_reclen d_off   d_name
                  2 directory    16         12 .
                  2 directory    16         24 ..
                 11 directory    24         44 lost+found
                 12 regular      16         56 a
             228929 directory    16         68 sub
              16353 directory    16         80 sub2
             130817 directory    16       4096 sub3

    Program source

        #define _GNU_SOURCE
        #include <dirent.h>     /* Defines DT_* constants */
        #include <fcntl.h>
        #include <stdio.h>
        #include <unistd.h>
        #include <stdlib.h>
        #include <sys/stat.h>
        #include <sys/syscall.h>

        #define handle_error(msg) \
               do { perror(msg); exit(EXIT_FAILURE); } while (0)

        struct linux_dirent {
            long           d_ino;
            off_t          d_off;
            unsigned short d_reclen;
            char           d_name[];
        };

        #define BUF_SIZE 1024

        int
        main(int argc, char *argv[])
        {
            int fd, nread;
            char buf[BUF_SIZE];
            struct linux_dirent *d;
            int bpos;
            char d_type;

            fd = open(argc > 1 ? argv[1] : ".", O_RDONLY | O_DIRECTORY);
            if (fd == -1)
               handle_error("open");

            for ( ; ; ) {
               nread = syscall(SYS_getdents, fd, buf, BUF_SIZE);
               if (nread == -1)
                   handle_error("getdents");

               if (nread == 0)
                   break;

               printf("--------------- nread=%d ---------------\n", nread);
               printf("inode#    file type d_reclen d_off   d_name\n");
               for (bpos = 0; bpos < nread;) {
                   d = (struct linux_dirent *) (buf + bpos);
                   printf("%8ld ", d->d_ino);
                   d_type = *(buf + bpos + d->d_reclen - 1);
                   printf("%-10s ", (d_type == DT_REG) ? "regular" :
                                    (d_type == DT_DIR) ? "directory" :
                                    (d_type == DT_FIFO) ? "FIFO" :
                                    (d_type == DT_SOCK) ? "socket" :
                                    (d_type == DT_LNK) ? "symlink" :
                                    (d_type == DT_BLK) ? "block dev" :
                                    (d_type == DT_CHR) ? "char dev" : "???");
                   printf("%4d %10lld %s\n", d->d_reclen,
                           (long long) d->d_off, d->d_name);
                   bpos += d->d_reclen;
               }
            }

            exit(EXIT_SUCCESS);
        }

SEE ALSO
        readdir(2), readdir(3), inode(7)

COLOPHON
        This page is part of release 5.05 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page,
        can be found at https://www.kernel.org/doc/man-pages/.

Linux                                                                              2019-03-06                                                                       GETDENTS(2)
我的Man  23年02月11日
